Just come across a nasty bug in version 8.4(6) that stops clientless VPN sessions from working.
Basically any session that needs to use extended plugins such as RDP won’t work following an upgrade – with a simple blank screen being displayed when clicking on the link.
Solution is to downgrade to 8.4(5) or upgrade to 9.1(2) or higher.
Cisco bug ID is CSCug23031
Intrinsic Network Solutions